Many issue areas of world politics are today governed by overlapping institutions that form global governance complexes. In this paper, we outline a framework that advances the emerging research agenda on institutional complexity in global governance in three directions. First, by identifying central dimensions along which systems of overlapping institutions vary—scale, diversity and density—we unite existing concepts of regime complexes, institutional complexes, polycentric governance and hybrid governance systems. Rather than competing concepts, we suggest these can be conceived as different manifestations of the same genus: a global governance complex (GGC). Second, by theorizing the effects of structural variation in GGCs for governance outcomes, we build a foundation for comparative research—across policy areas, regions and time—which may deepen our understanding of the causes and effects of institutional complexity in global politics. Third, methodologically, we discuss new quantitative and qualitative metrics which allow scholars to gauge dimensions of variation in institutional complexity. We conclude by outlining an agenda for future research on institutional complexity in global governance.